Field of the Invention
The present invention relates to a laser processing apparatus for performing laser processing for a workpiece such as an optical device wafer or a semiconductor wafer.
Description of the Related Art
In an optical device manufacturing process, an optical device wafer is formed by forming optical devices such as light emitting diodes or laser diodes in plural areas obtained by depositing a light emitting layer composed of an n-type semiconductor layer and a p-type semiconductor layer over a surface of a sapphire substrate, silicon carbide substrate, gallium nitride substrate, etc. having a substantially circular disc shape and partitioning the light emitting layer by plural streets formed in a lattice manner. Then, the optical device wafer is divided along the streets to manufacture the individual optical devices.
The cutting of the above-described optical device wafer along the streets is generally performed by a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table that holds a workpiece, a cutting unit for cutting the workpiece held by this chuck table, and a cutting feed unit that causes the chuck table and the cutting unit to relatively move. The cutting unit includes a rotating spindle, a cutting blade mounted on this spindle, and a drive mechanism that rotationally drives the rotating spindle. The cutting blade is composed of a base having a circular disc shape and a ring-shaped cutting edge mounted on the side-surface peripheral part of this base. For example, the cutting edge is formed to a thickness of about 20 μm by fixing diamond abrasive grains having a grain size of about 3 μm to the base by electroforming. Nevertheless, because the sapphire substrate, silicon carbide substrate, gallium nitride substrate, etc. to form the optical device wafer have high Mohs hardness, there is a problem that cutting by the above-described cutting blade is not necessarily easy and the productivity is low.
In order to solve the above-described problem, as a method for dividing an optical device wafer along streets, a laser processing method is attempted in which a pulsed laser beam having such a wavelength as to be transmitted through the wafer is used and the wafer is irradiated with the pulsed laser beam with the light focus point positioned inside the area along which the cutting should be performed. The dividing method using this laser processing method is the following technique. Specifically, a modified layer is continuously formed inside the wafer along streets by irradiating the wafer with the pulsed laser beam having such a wavelength as to be transmitted through the wafer along the streets from one surface side of the wafer with the light focus point positioned inside the wafer. Then, the wafer is divided by applying an external force along the streets whose strength is lowered due to the forming of the modified layer (refer to e.g. Japanese Patent Laid-open No. 2011-114018).
Furthermore, the following technique has been proposed as a method for dividing an optical device wafer along streets. Specifically, a laser processing groove is formed along the streets by irradiating the wafer with a pulsed laser beam having such a wavelength as to be absorbed by the wafer along the streets. Then, the wafer is divided by applying an external force along the streets along which the laser processing groove is formed (refer to e.g. Japanese Patent Laid-open No. 2008-311404).